Open AccessHigh‐speed Keck II and RXTE spectroscopy of Cygnus X‐2 – I. Three X‐ray components revealed by correlated variability
Author(s) -
O'Brien K.,
Horne Keith,
Gomer Richard H.,
Oke J. B.,
Van Der Klis M.
Publication year - 2004
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2004.07667.x
Subject(s) - physics , astrophysics , flux (metallurgy) , light curve , proportional counter , spectrograph , spectral line , spectroscopy , accretion (finance) , x ray , intensity (physics) , astronomy , optics , detector , materials science , metallurgy
We have performed simultaneous X‐ray and optical spectroscopic observations of the low‐mass X‐ray binary Cygnus X‐2. We have used a new data system attached to the Low Resolution Imaging Spectrograph (LRIS) instrument on Keck II to obtain spectra with a mean time resolution of 72.075 ms, simultaneous with pointed X‐ray observations using the Proportional Counter Array (PCA) onboard RXTE . In this paper, we have analysed the variability in both wavebands on time‐scales of 16 s. During our observations Cygnus X‐2 covered all three branches of the Z‐curve, allowing us to study how the changes in X‐ray spectral state affect the optical emission. As the optical flux rises, the X‐ray intensity first rises on the horizontal branch (0 < S z < 1) but then falls on the normal branch (1 < S z < 2) and flaring branch (2 < S z < 3) , where S z is a rank number characterizing the position on the Z‐curve. This linear increase in the optical flux with S z indicates the optical flux is a good predictor of the accretion rate (possibly normalized by its own long‐term average) inferred from the Z‐state S z . We have used this correlation to decompose the total X‐ray count rate into three distinct spectral components.